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fir/ 1.0 INTRODUCTION On behalf of Mr. Jay Mcllrath of Jay's Mini Mart, ATC Associates Inc. (ATC) has prepared this Workplan to conduct a subsurface soil vapor intrusion study,lf'located at 3302 North West Lane in Stockton, California (Site) (Figure 1). The Workplan also includes contaminant mass calculations and supplemental information specific to the receptor survey requested by the San Joaquin County Environmental Health Department (EHD) in correspondence dated October 1, 2008. 2.0 PURPOSE The purpose of the soil vapor intrusion study will focus on further evaluations to determine whether the Site poses a current or future health risk. Recent data analysis determined benzene concentrations in soil (soil boring SB-3) exceeded site-specific Tier 1 .. Environmental Screening Levels (ESLs). Additionally, benzene vapor intrusion calculations exceeded the Tier II target excess cancer risk (ECR) value of 1 E-06 when modeled exclusively using analytical soil concentrations in accordance with Department of Toxic Substances Control (DTSC) guidance (Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion To Indoor Air, Interim Final, DTSC revised February 7, 2005). Therefore, a subsurface soil vapor intrusion investigation through the collection of soil vapor samples is recommended. The following site-specific tasks are proposed: r • Complete four on-site shallow (four to five feet below ground surface [bgs]) soil borings; • Collect soil samples at the boring terminus, - • Install a soil vapor probe within each boring, and ' • Collect soil vapors samples from each vapor probe for laboratory analysis and subsequent vapor intrusion modeling. 3.0 SITE LOCATION -- The Site is located on the east side of North West Lane at the intersection of Alpine Avenue in Stockton, California, as shown on Figure 2. The property is located in a mixed residential, commercial, and industrial area and occupied by a retail gasoline fueling station r„ and convenience store. r 3.1 Site Background `, In 1997, two 8,000-gallon and one 10,000-gallon underground storage tanks (USTs) and associated piping and fuel dispensers were removed from the locations shown on Figure 3. _ Soil samples w re in r gitially collected from the bottom of the tank excavation basin at depths " ing-from]T 0-17"feet bgs. Soil samples were also collected from beneath the product dispensers and dispensing lines at depths of three to four feet bgs. Soil over-excavation activities were subsequently conducted on the northwest portion of the UST basin to a depth of approximately 24 feet bgs due to elevated levels of petroleum hydrocarbons. Analytical testing of the soil left in-place indicated detectable concentrations of petroleum hydrocarbon constituents and fuel oxygenates, including methyl tert-butyl ether (MTBE) in several samples. Approximately 400 cubic yards (cy) of soil were removed in conjunction with the r. Workplan to Conduct Subsurface Soil Vapor Intrusion Study \\// �� ATC Project 54.26874.0001 Page 1